[0001] The present invention relates generally to controllable inverter power supplies and,
in particular to a method and apparatus for providing a stabilized arc when GTAW welding.
[0002] Gas tungsten arc welding (GTAW), also called tungsten inert gas (TIG) welding, is
a well known method for welding metals, in particular aluminum and magnesium. A description
of GTAW, including the problems associated with welding aluminum and magnesium, may
be found in Welding Handbook. Chapter 3, Gas Tungsten Arc Welding, pages 74-107, 1991,
published by the American Welding Society.
[0003] It is well known in GTAW to use an AC square wave power source, which is particularly
suited for welding aluminum and magnesium. An AC square wave power source provides
a single phase output having a straight polarity (electrode negative) half-cycle and
reverse polarity (electrode positive) half-cycle, with a rapid transition between
half cycles.
[0004] In GTAW welding a square wave having independently adjustable duration of the two
half-cycles (positive and negative) is desirable. In other words, it is desirable
to have the positive half cycle duration be controllable to be longer or shorter in
time than the negative half-cycle. Hereinafter independent control of the positive
and negative half-cycle widths is called adjustable balance. It is also desirable
to independently control the amplitude of the two half-cycles, i.e. control the positive
half-cycle amplitude to be greater or less than the negative half-cycle amplitude
(hereinafter independent amplitude control).
[0005] One AC square wave power source is described in U.S. Patent No. 4,038,515, (the '515
patent) issued July 26, 1977 to Robert L. Risberg. The device disclosed in the '515
patent uses a single phase input and provides an AC square wave welding current output,
with adjustable balance, i.e. the duration of the electrode positive and electrode
negative half-cycles may be adjusted with respect to one another.
[0006] However, a power supply that uses a single phase input such as the power supply of
the '515 patent requires a very large single phase input current requirement. Such
a large single phase current draw is undesirable for industrial sites having three-phased
power because it unbalances the power lines.
[0007] There are prior art GTAW power supplies using an inverter with a three phase input
that have the capability of independent control of the output half-cycles. However,
many prior art GTAW inverter power supplies directly rectify the input line. Thus,
to design inverters for multiple input voltages operating directly from the power
lines requires compromises in the design, such as dual inverters with a series parallel
connection, or over current and over voltage ratings on the power components to be
re-linkable for the various inputs. Moreover, such designs provide an input power
factor that is highly capacitive. Such an input power factor can cause neutral unbalance
and line notching. Accordingly, it is desirable to provide an inverter for GTAW that
provides a constant secondary voltage, regardless of the input voltage and that provides
a relatively high power factor for a welding power source.
[0008] There are additional problems with known inverter GTAW power supplies. Inverter power
supplies for GTAW generally provide a substantially triangular wave shape load current.
Due to this triangular wave shape, in a welding application, or any other application
where wide range of output currents is required, it can be costly to size the output
inductor to both the minimum and maximum current required by the load, unless current
discontinuities can be accommodated.
[0009] For example, a welding power supply may be required to go from 20 amps or less to
300 amps or more. In order to accommodate a low current, an inductor must be very
large. It is costly to provide an inductor that will accommodate this wide range of
currents, without causing discontinuities in the output current. Moreover, a small
inductor is desirable for applications which require pulsing and output because a
large inductance will cause the slewing rate of the output to be slow, adversely affecting
the shape of pulsed or square wave outputs.
[0010] Unfortunately, the discontinuities in the output current will appear as an arc outage
which is generally not acceptable in GTAW. Accordingly, it is desirable to provide
an inverter power supply for GTAW that is capable of providing a wide range of output
currents, yet does not provide output current discontinuities and still does not require
the use of a costly large inductor.
[0011] When providing a GTAW inverter power supply it is desirable to provide the user with
the option of selecting straight polarity, reverse polarity or alternating polarity
welding. Such a machine has a wider range of applications and is more desirable to
the purchaser. It is well known in the prior art that it is difficult to start an
arc when the electrode is negative. Conversely, when the electrode is positive the
arc will start much easier. However, most DC GTAW is done with the electrode negative,
when it is difficult to start.
[0012] Accordingly, it is desirable to provide a GTAW inverter power supply that provides
an arc starting capability wherein the arc is started with a brief electrode positive
polarity followed by a rapid reversal to electrode negative polarity for the welding
process. Additionally, when using AC GTAW for the reasons stated above it is desirable
to provide an extended electrode positive period in order to facilitate arc starting.
[0013] It is also well known in the art that in AC GTAW the arc extinguishes with every
current reversal. When the reversal is in the direction of electrode emitting (from
electrode positive to electrode negative), the arc will re-ignite without much difficulty
because of the thermionic state of the electrode. However, when the reversal is to
the workpiece emitting (from electrode negative to electrode positive), arc re-ignition
problems are encountered. The workpiece is generally below thermionic temperature.
Also, the molten weld pool is very clean metal due to the ionic bombardment during
past electrode positive half cycles. Both of these factors contribute to difficulty
in reestablishing the electrode positive half cycle.
[0014] One way the prior art has compensated for this difficulty is by providing high frequency
overlays. However, there are many problems associated with such high frequency overlays,
one being that high frequency can provide undesirable noise which adversely affects
the operation of control circuits, telecommunication equipment and computer equipment.
Accordingly, it is desirable to provide a GTAW power supply that will provide for
arc starting (stabilizing) when trying to reestablish the electrode positive half
cycle without the use of superimposed high frequency.
[0015] An additional problem with inverter power supplies is that a considerable amount
of acoustic noise is generated by current reversal. Moreover, this acoustic noise
is greater for greater welding currents. Accordingly, it is desirable to provide a
GTAW power supply that will provide for a lower output current immediately before
current reversal.
[0016] Inverter power supplies will have a distributed (parasitic) inductance in the load
(from the weld cables and arc e.g.). This parasitic inductance will slow the transition
at current reversals, adversely affecting machine performance. A high forcing voltage
will cause a rapid current reversal. However, a high forcing voltage may cause damage
to switches in the inverter. Thus it is desirable to provide a forcing voltage sufficiently
high to cause a smooth and fast current reversal, but not so high as to damage switches.
[0017] Finally, it is well known that there is an additional safety factor with respect
to ventricular fibrillation, the major cause of electrocution, of DC when compared
to AC. This factor is given in IEC 479 as 3.75. Accordingly, it is desirable to provide
a GTAW power supply that provides a DC voltage at the terminals when not welding or
when the power source is at open circuit.
[0018] According to one aspect of the present invention a power supply for providing an
AC output to a load is provided. The power supply includes a first source of DC power
and an inverter that receives the DC power and provides an AC output to the load.
A second source of DC power is provided with at least one controllable switch that
connects the first and second sources of DC power in series with the load. In one
embodiment the power sources are connected in series with every output current reversal.
[0019] According to a second aspect of the invention a power supply for providing an AC
output to a load includes a transformer and a rectifier connected across the transformer.
A capacitor is connected across the rectifier to provide a substantially DC voltage.
A converter is connected across the capacitor to produce a substantially DC current
having a selectable magnitude as an output. An invertor receives the substantially
DC current and provides an AC output to the load. A second source of DC voltage is
provided and the converter includes a switch capable of connecting the substantially
DC voltage and the second source of DC voltage in series with the load.
[0020] A third aspect of the invention is a power supply for providing an AC output to a
load. The power supply includes a controllable source of DC current whose magnitude
is responsive to a current reference signal. A control circuit provides the current
reference signal, wherein the current reference signal is pulsed immediately prior
to the AC output reversing polarity.
[0021] Yet another aspect of the invention is a buck converter comprised of a source of
DC voltage provided between first and second buses. At least one switch having a control
input is connected to the first bus and a current gate is connected between the switch
and the second bus. At least one inductor is connected at one end to the node common
to the switch and the current gate, and the output of the converter is across the
second bus and the other end of the inductor. A variable frequency saw tooth wave
generator is connected to the control input, and the switch is turned on at the frequency
of the saw tooth wave. The frequency of the wave is chosen to be inversely related
to the desired current output magnitude.
[0022] In accordance with a final aspect of the invention an AC/DC GTAW power supply for
providing an output to an arc comprises a DC source of current and an inverter that
inverters the DC current to provide an AC output, or a DC output of either polarity
to the arc. In the event the user selects either an AC or DC electrode negative output
a period of electrode positive DC energy is provided at start up to help establish
the arc.
[0023] Embodiments of the present invention will now be described with reference to the
accompanying drawings, in which:-
Figure 1 is a block diagram of an AC/DC power supply constructed according to the
present invention;
Figure 2 is a circuit diagram of a preferred embodiment of the inverter power supply
constructed according to the present invention;
Figure 3A-3D depicts waveforms of the load current and switch state of an inverter
constructed according to the present invention;
Figure 4 is a circuit diagram of a preferred embodiment of the inverter power supply
constructed according to the present invention; and
Figure 5 shows waveforms of the reference current signal and the output current of
the circuit of Figure 4.
[0024] Before explaining at least one embodiment of the invention in detail, it is to be
understood that the invention is not limited in its application to the details of
construction and the arrangement of the components set forth in the following description
or illustrated in the drawing. The invention is capable of other embodiments or being
practised or carried out in various ways. Also, it is to be understood that the phraseology
and terminology employed herein is for the purpose of description and should not be
regarded as limiting.
[0025] The present invention is directed primarily to an inverter power supply for use in
AC or DC GTAW welding. Referring to Figure 1, a block diagram of a preferred embodiment
of a GTAW power supply 100 is shown. Additionally, waveforms between and within various
stages of GTAW power supply 100 are shown.
[0026] As will be described in detail below, GTAW power supply 100 provides an output suitable
for AC or DC GTAW welding. In the AC mode GTAW power supply 100 provides an output
having adjustable balance and independent amplitude control. Moreover, even at low
currents discontinuities in the load are avoided. Also, as the current reverses from
one polarity to the other, a burst of energy is provided to help re-establish the
arc and avoid arc instability. Similarly, in the DC electrode negative mode GTAW power
supply 100 provides a short burst of electrode positive current to start the arc.
[0027] As shown on Figure 1 an input 60 Hz AC supply 101 is provided to a transformer stage
102. Transformer stage 102 includes a plurality of main secondary windings S1-S3 and
a clamp secondary winding S4. Input supply 101 is a high voltage, low current source
and is transformed to a 60 Hz AC low voltage, high current, output 105 through main
secondary windings S1-S3. Of course as one skilled in the art will recognize the frequency
may be something other than 60 Hz for both the input and the output.
[0028] Low voltage, high current, output 105 is provided to a rectifier stage 106 which
includes a diode bridge 107 and a capacitor 108. Diode bridge 107 receives 60 Hz low
voltage, high current output 105 and rectifies it to a rectified DC signal 109. Rectified
DC signal 109 is received by capacitor 108, which provides a filtered DC signal 110
having a magnitude of approximately 85 V. In practice capacitor 108 may be more than
one capacitor, reducing costs and improving performance. In alternative embodiments
transformer stage 102 and rectifier stage 106 may be replaced by other components
which provide a source of DC current.
[0029] Filtered DC signal 110 is received by a buck converter stage 111 (also called a switching
regulator) which includes a switch 112 and an inductor 113. Switch 112 receives filtered
DC signal 110 and provides a chopped DC signal 114, preferably having a frequency
in the tens of Khz range. As one skilled in the art should recognize, the frequency
of chopping could vary widely, depending upon the particular application needed and
the components used to perform the switching. Chopped DC signal 114 is provided to
inductor 113 which provides a smoothed DC signal 116 as an output. In practice switch
112 may be implemented using a variety of semiconductor switches, including MOSFETs,
and inductor 113 may be more than one inductor. Smoothed DC signal 116 has a variable
magnitude, depending upon the magnitude of filtered DC signal 308 and the switching
of buck converter stage 111.
[0030] Smoothed DC signal 116 is provided to an inverter stage 117, which includes IGBTs
or other semiconductor switches in the preferred embodiment. Inverter stage 117 inverts
its input and provides an AC output 118, having variable frequency, adjustable balance
and independently controlled amplitude. The frequency and width of output 118 are
controlled by the switching of inverter stage 117. The magnitude of output 118 is
dependent on the magnitude of smoothed DC signal 116, which in turn is controlled
by the switching of buck converter stage 111. When operating in the DC mode output
inverter stage 117 is controlled to provide a constant polarity DC output.
[0031] The magnitude, duration and frequency of output 118 is sensed by a sense circuit
119. Sense circuit 119 provides that information to a control circuit 120. Control
circuit 120 provides switching signals to switch 112 to control the magnitude of smoothed
DC signal 116 and, consequently, output 118. Control circuit 120 also provides switching
signals to inverter stage 117 to control the frequency and balance of output 118.
[0032] Clamp secondary S4 is provided to help protect stage 117. Clamp secondary S4 transforms
input 101 to a 120 volt, low current signal and provides it to a clamp stage 121.
The output of clamp stage 121 is a filtered DC signal of about 165 volts. As will
be explained in detail below with respect to Figure 4, clamp stage 121 effectively
clamps the voltage applied to IGBTs in inverter stage 117 to a safe level. Thus, clamp
stage 121 protects IGBTs in inverter stage 117.
[0033] Referring now to Figure 2, a circuit diagram of one embodiment of GTAW power supply
100 is shown. The embodiment of Figure 2 has the stages of Figure 1, except clamp
secondary S4 and clamp stage 121. Sense circuit 119 and control circuit 120 are used
in the embodiment of Figure 2, but are not shown.
[0034] The circuit of Figure 2 includes transformer stage 102, rectifier stage 106, buck
converter stage 111, and inverter stage 117. Transformer stage 102 includes a transformer
T1, which has a plurality of windings Pl-P3 and secondary windings S1-S3. Transformer
stage 102 is designed with some primary/secondary leakage reactance in order to obtain
a high power factor. For example, using a primary/secondary leakage reactance in the
range of 1.0-1.5 millihenrys at 230 V, a power factor of .94 is obtained. Preferably,
a power factor of at least 0.90 is obtained.
[0035] Primary windings Pl-P3 receive as an input three phase input signal 101 (Figure 1)
on a plurality of input terminals V1-V3. As shown in Figure 2, a three-phase input
and three-phase transformer is utilized. However, as one skilled in the art will readily
recognize a single-phase input and single-phase transformer could also be used.
[0036] Secondary windings S1-S3 are designed to produce an AC output of 60 V each. GTAW
power supply 100 can be designed for any of a variety of standard voltages by providing
a different primary winding depending upon the input voltage. For example, the primary
windings may be designed to accept any voltages, such as 200, 220, 230, 380, 400,
415, 460, 480 and 575, and any frequency such as 50 or 60 Hz. Indeed, the primary
could have capped windings or extended windings such that the same transformer can
accommodate multiple input voltages.
[0037] The output of secondary windings S1-S3 is provided to rectifier stage 106 which includes
diode bridge 107 and capacitor 108. Diode bridge 107 includes 6 diodes 207 and capacitor
108 has, in the preferred embodiment, a large value (24,000 mfd) and is charged to
85 V. Capacitor 108 filters the DC rectified signal to produce a filtered DC signal
of about 85 v between the positive bus and negative bus.
[0038] The output of rectifier stage 106 (the positive bus and negative bus) is provided
as the input to the buck converter stage 111, sometimes called a chopper. In this
embodiment buck converter stage 111 is comprised of a switch (MOSFET) Q1, a diode
D4 and an inductor L1. As one skilled in the art will recognize other electronic components
having the ability to control current flow (a current gate) may be used in place of
MOSFET Q1 and/or diode D4.
[0039] Buck converter stage 111 performs a function analogous to a DC transformer. In other
words it receives as an input the 85 V DC supply and provides a substantially DC output
having a controllable adjustable magnitude. MOSFET Q1 is switched on and off at a
frequency much higher than the line frequency. In one unique embodiment, this frequency
varies from 24 kHz at high current demand to 60 kHz at low current demand. The relationship
between the switching frequency of MOSFET Ql and the output current will be explained
in greater detail below.
[0040] Generally speaking the buck regulator controls the current by pulse with modulation
(PWM). In a PWM controller the output is a function of the ratio of on time to off
time at the given frequency. If this on time to off time ratio is large, the PWM will
produce high power. If it is low, it will produce low power.
[0041] The output of buck converter stage 111 is provided to inverter stage 117. Inverter
stage 117 comprises a plurality of switches Q3-Q6, and provides its output to a load
208. To understand the operation of buck converter stage 111 it is necessary to consider
the state of the switches in inverter stage 117. For example, assume that switches
Q3 and Q6 are on, thereby providing current to load 208. When MOSFET Ql is turned
on the 85 V bus will be switched across diode D4. Diode D4, of course, is blocking,
thus this voltage appears across switches Q3-Q6 (IGBTs in the preferred embodiment).
Because switches Q3 and Q6 are on, positive current will flow through switch Q3, arc
load 208, switch Q6 and inductor Ll. If inductor Ll was not present the current flow
would be approximately equal to 85 V divided by the load impedance. In the case of
an arc load, this impedance is very low, thus the current would be extremely high
and essentially out of control.
[0042] Thus, the function of inductor Ll is to buck this current and, while doing so, store
energy. By bucking the current, the current flow through load 208 is controlled and
regulated to a relatively slow rising value. Also, as the current increases a magnetic
field is induced in inductor Ll, thus storing energy therein. In other words, inductor
Ll filters pulse width modulated voltage pulses supplied by MOSFET Q1, and provides
predetermined rising and falling current slopes to the arc through output inverter
stage 117.
[0043] When Ql is switched on, the 85 V bus is connected to the arc load in series through
inductor Ll and output inverter stage 117. The filtering function of Ll will cause
the output current to change at a rate equal to the forcing voltage applied to Ll,
multiplied by the on time of MOSFET Ql, divided by the inductance value of Ll. Thus,
the current will continue to rise so long as MOSFET Ql stays on.
[0044] MOSFET Ql is turned off after a time determined by the desired load current flow
and the actual load current flow. When MOSFET Ql is switched off, the arc or load
current will continue to flow, but will be diverted through free-wheeling diode D4,
and again the filtering function of Ll will cause the arc current to decrease at a
rate determined by the forcing voltage applied to Ll, multiplied by MOSFET Ql off
time, divided by the inductance of inductor Ll. The forcing voltage applied to inductor
Ll when MOSFET Ql is off is the arc or load voltage plus all series voltage drops
in the current path. When MOSFET Ql is off, the current flows from point A through
diode D4, switch Q3, load 208, switch Q6, and back to inductor Ll.
[0045] In summary, as long as MOSFET Ql is on the magnitude of the output current increases,
and as long as switch Ql is off the magnitude of the output current decreases. Thus,
by turning MOSFET Ql on and off at regular intervals, it is possible to regulate the
load current.
[0046] The control of MOSFET Q1 utilizes a saw tooth wave, a feedback signal, and a reference
current signal. A saw tooth wave signal is generated in accordance with well known
prior art, and is used to turn on MOSFET Q1 at regular intervals. Also, for any given
desired output current, a reference current signal is established. The reference current
signal is compared to a feedback signal representing actual current to generate an
error signal. MOSFET Q1 is then turned off whenever the output current reaches a predetermined
value as determined by the reference current signal compared to the load current signal.
[0047] As one skilled in the art will recognize, this load current will have somewhat of
a triangular wave shape, rising during the time MOSFET Q1 is on and falling during
the time MOSFET Q1 is off. The amplitude of the rise and fall can be determined by
the switching frequency of MOSFET Q1, the on time to off time ratio of MOSFET Q1,
the size of the inductor L1, the source voltage and the load voltage.
[0048] The triangular wave shape may be seen in Figure 3, which shows the state of MOSFET
Q1 and the output current through load 208 for both high and low output currents.
Figure 3A shows the state of MOSFET Q1 for a high output current. It may be seen that
switch Q1 is on for a relatively large portion of time (approximately 75 percent of
each complete cycle). As shown in Figure 3B the corresponding output current is relatively
high and rises during the time switch Q1 is on, and falls during the time switch Q1
is off.
[0049] Similarly,, Figures 3C and 3D show the state of switch Q1 and the output current
for a low output. It may be seen in Figure 3C that switch Q1 is off for a relatively
large portion of time (approximately 85% of the time). It should be noted that as
shown in Figure 3 the time of one complete cycle (on time plus off time) of switch
Q1 is the same for high and low currents. In one embodiment the switching frequency
of switch Ql will vary, depending upon the magnitude of the output current.
[0050] As shown in Figure 3D the rise time is sufficiently less than the fall time for such
a low output current, so the output current reaches zero prior to the switching on
of switch Ql, causing current discontinuities in load 208. Such current discontinuities
are common in the prior art and are not acceptable for welding applications.
[0051] In accordance with one embodiment of the present invention the switching frequency
of switch Q1 is increased as the current decreases to prevent such current discontinuities.
The frequency is determined through practice when considering the desired output,
the inductor size, the conduction losses in switch Q1, the switching losses of switch
Q1, losses of the snubbers that are required for switching of switch Q1, the ripple
tolerance in load current, and other well known engineering factors.
[0052] The inventors have, through practice, determined that a design using an inductor
L1 of size between 1.0 and 1.5 millihenrys and an 85 V supply bus the following frequency
versus current parameters are suitable:
LOAD CURRENT |
SWITCH Q1 FREQUENCY |
1 AMP |
60 kHz |
100 AMPS |
50 kHz |
200 AMPS |
40 kHz |
300 AMPS |
30 kHz |
400 AMPS |
20 kHz |
[0053] The embodiment of Figure 2 is able to provide either polarity DC output, or an AC
output with adjustable balance and independent amplitude control. It is readily seen
how this may be accomplished by looking at the inverter switches Q3-Q6 and buck converter
stage 111.
[0054] The load current flows in one direction when switches Q3 and Q6 are on (right to
left on the schematic of Figure 2), and in the opposite direction when switches Q4
and Q5 are on. There is no limit on how long switches Q3 and Q6 or switches Q4 and
Q5 may conduct; the only limitation is that neither switches Q3 and Q5, nor switches
Q4 and Q6, may be on together. In other embodiments the switches may be on together
for a brief period of time. Thus, if a DC output is desired either switches Q3 and
Q6 are left on and switches Q4 and Q5 off, or visa versa, depending upon the polarity
desired.
[0055] An AC output is obtained by turning switches Q3 and Q6 on and switches Q4 and Q5
off, and then reversing the process. The width of each half cycle is determined by
the length of time the switches are on and off. It is readily seen that there is no
need to control switches Q3 and Q6 such that they are on the same length of time switches
Q4 and Q5 are on. Accordingly, an output having adjustable balance is provided. In
the preferred embodiment it is desirable to have the AC adjustable from 40 to 400
Hz, although other frequencies are easily obtainable.
[0056] Similarly, the output can be made asymmetrical with respect to current. If the reference
current signal that controls switch Ql is set so that it has one value when switches
Q3 and Q6 are conducting and another value when switches Q4 and Q5 are conducting,
the load current magnitude may be separately controlled on each half cycle of AC output.
This makes the output independently controllable on each half cycle with respect to
both duration and amplitude of each half cycle.
[0057] The fast and controllable switching of this design also makes it possible to utilize
the thermionic emission characteristics of electrode positive to start AC welding.
Even if the DC electrode negative welding is desired, inverter stage 117 can be controlled
to provide a short burst of electrode positive to very quickly heat the electrode
until it becomes thermionic, and then switch to electrode negative after the arc has
been established for heat flow to the work to obtain a penetrating weld. Similarly,
the control can be used to enhance starts of AC arcs by always bringing on an extended
period of electrode positive time to establish the arc. The electrode positive time
is determined by factors such as tungsten, size, geometry, torch type and material
thickness.
[0058] Figure 4 shows a circuit diagram of an alternative embodiment 400 implementing the
block diagram of Figure 1. Transformer stage 102 includes three primary windings Pl-P3
and three secondary windings Sl-S3. As in the configuration of Figure 2 primary windings
P1-P3 are connected to a three phase voltage input V1-V3, and secondary windings Sl-S3
produce a 60 V AC output. Also as in the embodiment of Figure 2, rectifier stage 106
includes a rectifier bridge 107 comprised of six diodes 207, and a capacitor 108.
The output of rectifier stage 106 is a filtered DC output as shown at 110 of Figure
1, and is provided as the positive and negative bus.
[0059] In the embodiment of Figure 4, buck converter stage 111 is separated into two circuits.
This limits the number of MOSFETS that need to be in parallel to handle the necessary
current. Accordingly, buck converter stage 111 includes a pair of inductors L2 and
L3, a pair of switches Q2A and Q2B, a snubber circuit for switch Q2A comprising a
resistor Rl, a diode Dl and a capacitor Cl, and a snubber circuit for switch Q2B comprising
a resistor R2, a diode D2 and a capacitor C2. In one embodiment switches Q2A and Q2B
each consist of three parallel MOSFETS.
[0060] Switches Q2A and Q2B are preferably MOSFETS, but may be any suitable switching device,
and chop the positive and negative bus to provide a chopped DC signal in the kilohertz
range, just as MOSFET Ql functioned in the embodiment of Figure 2. The chopping (switching
on and off) is timed with a saw tooth oscillator, a feedback signal and a reference
current signal to control the on and off time of switches Q2A and Q2B to keep the
magnitude of smoothed DC signal 116 at the desired level. Switches Q2A and Q2B have
separate sawtooth oscillators that are synchronized to be at 180 electrical degrees
from one another. This has the advantage of reducing the ripple current in the arc
to 50% or less of the ripple currents found in either inductor L2 or L3.
[0061] Also as previously described in conjunction with Figure 2, the chopping frequency
may be variable, and depend upon the desired output current. Inductors L2 and L3 together
function as inductor Ll did in the embodiment of Figure 2. They buck the current to
control and limit it, and while doing so store energy to be returned when switches
Q2A and Q2B are turned off.
[0062] As in Figure 2, the buck converter stage 111 provides smoothed DC signal 116 to inverter
stage 117. Inverter stage 117 is identical to that shown in Figure 2, with the addition
of a capacitor C5 to protect switches Q3-Q6 from excessive dv/dt. Switches Q3-Q6 switch
the current across the load as in the embodiment of Figure 2, to provide an AC output
having an adjustable balance and independent amplitude control, or a DC output of
either polarity.
[0063] Even utilizing the variable frequency chopping as described in conjunction with Figure
2 it is possible for inductors L2 and L3 to "run out" of energy at low current (50
amps or less e.g.) and cause a current discontinuity. One way to prevent this is to
pump up inductors L2 and L3 just prior to polarity change. This may be accomplished
by creating a relatively high pulse of current just before polarity change (called
an increasing pre-commutation pulse). In the embodiment of Figure 4 the "pumping up"
is accomplished by creating a pulse in the reference current signal corresponding
to about 50 amps, that starts 250 microseconds prior to polarity reversal. The pulse
in the reference current signal causes buck converter stage 111 to try and create
a 50 amp pulse in the output current.
[0064] However, as one skilled in the art will recognize inductors L2 and L3 will cause
this pulse to have a relatively slow rise time, but during this current rise considerable
energy is stored in inductors L2 and L3. Indeed, the magnitude and width of the pulse
are chosen so enough energy is stored in inductors L2 and L3 to prevent current discontinuity
at output currents as low as 1 amp. Of course, other minimum thresholds could be selected.
[0065] Another benefit of providing such a current pulse is that it will create a similar
initial current pulse after the current reversal, thereby assisting re-ignition of
the next half cycle. As stated above, the inventors have determined that for the embodiment
of Figure 4 a pulse of 250 microseconds is sufficient. This is very short when compared
to the 40 to 400 Hz (25 - 2.5 milliseconds) switching frequency of Q3-Q6 and the output
current. Thus, this increasing pre-commutation pulse adds very little to the RMS value
of the output current.
[0066] Referring now to Figure 5 the effect of pumping up inductors L2 and L3 is graphically
shown. Figure 5A shows the output of buck converter stage 111, with the 50 amp pulse
provided at the end of each load cycle. As can be seen on Figure 5A a current reference
signal corresponding to a 1 amp output level is maintained until just prior to the
current reversal. Then a narrow pulse in the current reference signal that corresponds
to a 50 amp output is provided. As shown in Figure 5B, the current reference signal
pulse causes a current spike to be applied to the load immediately prior to reversal.
A similar current spike is also created after load current reversal. Thus, the reference
current signal pulse helps avoid output current discontinuities and the ensuing opposite
polarity output spike helps re-ignite the arc.
[0067] In accordance with another aspect of the present invention, the reference current
is pulsed to a lower magnitude when the power supply is delivering a high load current.
As stated above, at high load currents, the acoustic noise caused by current reversals
may be significant. Because the acoustic noise is dependent upon the magnitude of
the output current, it is possible to decrease the noise by "pulsing" down (called
a decreasing pre-commutation pulse) the reference current signal immediately prior
to current reversal. As in the case of the increasing pre-commutation pulse applied
above to "pump up" the inductor, this decreasing pre-commutation pulse will have very
little effect on the RMS value of the output current. Moreover, because the output
current is a high current, the problems associated with arc rectification at reversal
discussed above with respect to low output currents are not present.
[0068] The inventors have determined that a pulse width of 270 microseconds is sufficient
to reduce acoustic noise. Additionally, the reference current is pulsed to a level
corresponding to 50 amps. However, for high outputs (300 amps or more), the output
current will decrease, but not all the way to 50 amps, due to slewing in the power
supply.
[0069] During current reversal through the load, inductors L2 and L3 oppose the distributed
inductance in the load, such as that found in cables, the arc and coupling coils.
This causes inductors L2 and L3 to produce a high voltage that is applied across the
load and IGBTs Q3-Q6. While the voltage aids in reversing the current, if left unchecked
a high voltage could damages IGBTs Q3-Q6. For example, IGBTs having a voltage rating
of 600 V are typical, but the voltage briefly generated by inductors L2 and L3 to
oppose the distributed load inductance may easily surpass 600 V. Thus, another feature
of the present power supply is that a high forcing voltage is provided to aide at
current reversal, but the voltage is clamped to a safe value.
[0070] Clamp stage 121 and clamp secondary winding S4 were provided to protect IGBTs Q3-Q6
from such an over voltage. Clamp secondary winding S4 on transformer T1 provides a
120 V AC output. The 120 V output is provided to clamp stage 121 to produce a rectified
165 volt output. Clamp stage 121 includes diodes D7-D10 arranged in a bridge configuration
to rectify the transformed AC input signal. The 165 volt rectified output is provided
to a capacitor C4, which charges to provide a 165 v DC filtered signal.
[0071] The positive side of capacitor C4 is connected to the negative bus of power supply
400. Thus, the negative side of capacitor C4 will be approximately -165 V (after charging)
with respect to the negative bus of power supply 400. Under conditions other than
a current reversal, a diode D3 blocks the voltage on capacitor C4 from being applied
to the arc load or the inverter stage 117. When a reversal of the arc occurs, the
165 V DC signal is stacked with the 85 V DC bus to clamp the voltage across inductors
L2 and L3, and thus across IGBTs Q3-Q6 to approximately 240 V. Thus, the voltage is
sufficiently high to stabilize the arc, and not so high as to damage IGBTs Q3-Q6.
[0072] In operation, if switches Q3 and Q6 are conducting with current flowing from the
electrode to work as when welding, inductors L2 and L3 would have current flow in
their windings creating a magnetic field and a level of stored energy therein. If
switches Q3 and Q6 are turned off, inductors L2 and L3 will act in concert to continue
a current flow. Because the current path through the arc is no longer available, inductor
Ll will try to push current from point A through diode D4, to the positive bus, through
capacitor 108 to the negative bus, and through capacitor C4 and diode D3 and back
to point B. Similarly, inductor L2 will try and push current from point C through
a diode D5, capacitor 108, capacitor C4, diode D3 and back to point B. This drives
the voltage at point b down. When the voltage at point B decrease more than one diode
drop below the voltage at the negative side of capacitor C4 (about 250 volts), diode
D3 becomes forward biased and the voltage across IGBTs Q3-Q6 is clamped at 250 volts.
[0073] In the preferred embodiment capacitor 108 has a large value (24,000 mfd) and is charged
to 85 volts. Capacitor C4 has a much smaller value (preferably approximately 1,000
mfd) but is charged to approximately 165 volts. As stated above when diode D3 is conducting
capacitors 108 and C4 are in series and connected across point B and the positive
bus. This advantageously uses the supply voltage as part of the clamp voltage.
[0074] In practice a five microsecond delay between turning off one pair of transistors
Q3-Q6 and a turn on of the next pair of transistors is used. The five microsecond
delay accommodates the lumped circuit distributed impedance and resonances. It also
allows time for switches Q2A and Q2B to respond to the new current reference signals.
[0075] Because it is difficult to balance the stored energy in inductors L2 and L3 throughout
the range of the output current level, capacitor C4 will have a tendency to pump up
to a value that could exceed component ratings. Therefore, a switch Q7 and a resistor
R3 are provided to limit the voltage on capacitor C4. This circuit allows capacitor
C4 to pump up to approximately 270 v. When the voltage exceeds 270 V across capacitor
C4, voltage sense circuitry will cause switch Q7 to conduct through resistor R3 and
reduce the voltage to about 230 V. Additionally, the decreasing pre-commutation pulse
described above reduces energy build up on capacitor C4. This helps reduce the amount
of wasted power when switch Q7 is on and heat is dissipated in resistor R3.
[0076] As one skilled in the art should recognize, unless inductors L2 and L3 are acting
as voltage sources diode D3 will be back biased. Thus, the present invention will
only produce the increased (but clamped) re-strike voltage when inductors L2 and L3
have been conducting current, i.e only when the arc has been present. This is a great
advantage because it can produce a pseudo open circuit voltage of nearly 250 V while
operating in a loaded or arc on condition, but will revert rapidly to the safe value
of 85 v DC when there is no arc. Moreover, because a DC voltage is always applied
to the terminals when not welding or when the power source is at open circuit voltage,
the present invention takes advantage of the additional safety factor of DC when compared
to AC.
1. A power supply for providing an output to a load (208) comprising:
a first source (106) of DC power having a first non-zero voltage;
a converter (111) in electrical communication with the source (106) of DC power and
capable of being connected to the load (208);
a second source (121) of DC power having a second non-zero voltage; and
at least one controllable switch (112), capable of connecting the first (106) and
second (121) sources of DC power, in an additive, and non-parallel arrangement across
the input of the converter (111), wherein the voltage applied to the converter input
is the sum of the voltages of the first (106) and second (121) sources of DC power.
2. The power supply of claim 1, wherein the first source (106) of DC power has a first
voltage magnitude and the second source (121) of DC power has a second voltage magnitude,
wherein the second voltage magnitude is greater than the first voltage magnitude.
3. The power supply of claim 1 or claim 2, including a control circuit (120), wherein
the switch (112) is controlled to connect the first (106) and second (121) sources
of DC power in series with the load (208) at every current reversal of the output.
4. The power supply of any one of the preceding claims, wherein:
the first source (106) of DC power includes a buck converter (111) comprising the
controllable switch (112) and at least one inductor (L1);
a source of DC voltage having a positive and a negative bus is provided as an input
to the buck converter (111); and
the switch (112) is connected between the at least one inductor (L1) and one of the
voltage source buses.
5. The power supply as claimed in claim 4, wherein the source of DC voltage comprises
a three phase transformer (102), a rectifier (106) connected across the transformer
(102), and at least one capacitor (C3) connected across the rectifier (106).
6. The power supply of claim 4 or claim 5, wherein the buck converter (111) provides
a current output of adjustable magnitude, and the switch (112) has a control input,
further wherein a variable frequency saw tooth wave generator (120) is connected to
the control input, whereby the switch (112) is turned on at the frequency of the saw
tooth wave, and further wherein the frequency is inversely related to the desired
current output magnitude.
7. A power supply for providing an ac output to a load (208) comprising:
a converter (111) having a first source (106) of non-zero DC voltage;
an inductor (L1) in series with the converter (111);
a second source (121) of non-zero DC voltage; and
a switch (112) capable of connecting the first (106) and second (121) sources of DC
voltage in an additive, non-parallel arrangement across the input of the converter
(111) , wherein the voltage across the converter (111) is clamped by the first (106)
and second (121) DC sources of voltage, and wherein the voltage applied to the converter
(111) input is the sum of the voltages of the first (106) and second (121) sources.
8. The power supply of claim 7, wherein the second (121) source of DC voltage comprises:
a secondary winding (S4) on a transformer (102);
a rectifier connected across the secondary winding (S4); and
a capacitor (C4) connected across the rectifier.
9. The power supply of claim 11, wherein the switch (112) is a diode (D3) and connects
the capacitor (C4) in series with the first (106) source of DC voltage.
10. The power supply as claimed in any one of claims 7 to 9, including a three phase transformer
(102) having a leakage reactance sufficient to provide a power factor of at least
0.9, the transformer providing an output to a rectifier (106) having an output that
is the first source of DC power.